Chemists Thread Eight Molecular Helices Into Single Nanographenes

Researchers have unveiled a modular stereoselective synthetic strategy that allows them to control, with unprecedented precision, the handedness of multiple helicene units embedded within large polycyclic aromatic hydrocarbons. The work, published in Nature Synthesis, demonstrates the preparation of hexabenzocoronenes and graphene nanoribbons carrying up to eight uniformly aligned helicene motifs, each installed with defined stereochemistry. The achievement opens a practical route to a new family of chiral nanocarbons whose optical and electronic properties can be tuned systematically, a long-sought goal in the field of molecular materials.

Helicenes are ortho-fused polycyclic aromatic compounds that adopt a non-planar, screw-shaped geometry. Because the two possible screw senses, designated P for right-handed and M for left-handed, are mirror images of one another, individual helicenes are inherently chiral even though they contain no classical stereogenic center. When several helicene units are woven into a single nanographene scaffold, the relative arrangement of their screw senses matters enormously. An arrangement in which all helices turn the same way produces a molecule with strong chiroptical activity, while a mixed arrangement can partially or fully cancel the chiral response. Until now, preparing these multi-helical architectures with complete stereochemical control has been a formidable synthetic challenge, because forming each helical rim typically generates a mixture of stereoisomers that are difficult to separate.

The new strategy addresses this problem through a modular design in which stereodefined building blocks are assembled stepwise around a central carbon-rich core. By carefully orchestrating the sequence of cyclization reactions, the chemists can dictate whether each helicene adopts the P or M configuration as it forms, rather than relying on statistical mixtures. This converts what was previously an uncontrolled combinatorial problem into a rational synthesis. The approach proved general enough to furnish hexabenzocoronene derivatives, a class of compact disk-like nanographenes, as well as extended graphene nanoribbon segments decorated with up to eight helicene units all twisting in the same direction.

The significance of achieving uniform helix alignment extends well beyond synthetic elegance. Chiral nanographenes interact differently with left- and right-circularly polarized light, producing large circular dichroism signals and, in some cases, circularly polarized luminescence. Materials that emit or absorb circularly polarized light efficiently are of intense interest for next-generation display technologies, optical data storage, spin-selective electronics, and chiral photonics. The intensity of these effects generally scales with the degree of chiral order in the molecule, so a nanographene bearing eight cooperatively aligned helices can display chiroptical responses far stronger than those of a molecule containing a single helicene.

A particularly striking feature of the reported materials is their near-infrared emissive behavior. Near-infrared emission, roughly in the 700 to 1700 nanometer window, is valuable for biological imaging, telecommunications, and optical sensing, yet most organic molecules emit at shorter wavelengths. Rigid, planarized nanocarbon frameworks tend to reduce the energy gap between their ground and excited states, pushing emission toward longer wavelengths, and the appended helicenes in this work appear to contribute both to the red shift and to the chiral character of the emitted light. Because the synthetic strategy is modular, the researchers can adjust the number, position, and handedness of the helicene units and thereby tune the photophysical and chiroptical properties in a predictable manner rather than by trial and error.

The synthetic logic rests on established carbon-carbon bond-forming reactions, notably oxidative cyclodehydrogenation, which fuses adjacent aromatic rings into the extended graphene-like lattice, combined with diastereoselective steps that lock in the helix handedness. In a diastereoselective transformation, the existing stereochemical information within an intermediate biases the formation of a new stereogenic element toward one outcome. In these nanographenes, the first helix to form sets the conformational stage for its neighbors, and subsequent cyclizations follow the templating influence of the pre-existing helical rims. Enantioselective steps, in contrast, distinguish between the two mirror-image pathways from the outset, allowing the chemists to choose globally left-handed or globally right-handed products. Combining both forms of stereocontrol in one sequence is what makes the construction of up to eight aligned helices feasible.

Multi-helical nanographenes of this kind can be described in terms of their diastereomeric relationships: molecules with the same set of helix configurations but different relative arrangements are diastereomers, while a fully right-handed molecule and its fully left-handed counterpart are enantiomers. Distinguishing and characterizing these species requires sophisticated analytical tools. The researchers relied on standard structural confirmation methods common to the field, including nuclear magnetic resonance spectroscopy to probe the aromatic framework, mass spectrometry to confirm molecular formulas, high-performance liquid chromatography on chiral stationary phases to separate and quantify enantiomers, and circular dichroism spectroscopy to measure the chiroptical signatures. Photoluminescence measurements then quantified how efficiently the materials emit light and at which wavelengths.

The broader context of this work is the growing field of chiral nanocarbons, which includes helically twisted graphene nanoribbons, chiral carbon nanotube models, and saddle-shaped or bowl-shaped polycyclic aromatics. These structures bridge molecular chemistry and materials science: they are discrete, well-defined molecules that can be purified and characterized like ordinary compounds, yet their extended pi-conjugated frameworks give rise to the electronic behavior of graphene, one of the most celebrated materials of the past two decades. Introducing controlled chirality into such frameworks adds a functional dimension that achiral nanographenes lack, enabling interactions with polarized light and with chiral biological or chemical environments.

One of the enduring obstacles in this area has been scalability and stereochemical purity. Many beautiful chiral nanocarbon structures have been reported only as racemic mixtures, in which left- and right-handed forms coexist, limiting their usefulness in devices that exploit circularly polarized light. Others required laborious chiral chromatographic resolution that yields only tiny quantities of material. A stereoselective synthesis that directly delivers a single enantiomer or a defined diastereomer, in modular fashion, represents a meaningful step toward practical exploitation. It also allows systematic structure-property studies, since a family of closely related stereoisomers can be compared to isolate the influence of each helical unit on absorption, emission, and chiroptical response.

Looking forward, the modular nature of the strategy suggests extensions in several directions. Varying the substituents on the helicene rims could modulate solubility, packing in the solid state, and supramolecular assembly, all of which are critical for processing these large molecules into thin films for devices. Incorporating heteroatoms such as nitrogen, boron, or oxygen into the nanographene core could further tailor the electronic structure, potentially yielding ambipolar or n-type chiral semiconductors. Longer graphene nanoribbons with patterned helicene arrays might display anisotropic charge transport combined with strong circular dichroism, a combination relevant to chiral spintronics. And because the emission can reach the near-infrared, biomedical applications that demand deep tissue penetration and chiral light-matter interactions become plausible, provided the materials can be made biocompatible. For now, the immediate achievement is conceptual: demonstrating that multiple helical elements in a single nanographene can be installed with deliberate, uniform stereochemistry, converting a class of molecules once accessible only as complicated mixtures into rationally designed, property-tunable materials for advanced optoelectronics.

Subject of Research: Enantio- and diastereoselective synthesis of multi-helical nanographenes

Article Title: Enantio- and diastereoselective synthesis of multi-helical nanographenes

Article References: Shimizu, R., Nogami, J., Kishida, Y., Morita, F., Maeda, C., Uekusa, H., & Tanaka, K. (2026). Enantio- and diastereoselective synthesis of multi-helical nanographenes. Nature Synthesis. https://doi.org/10.1038/s44160-026-01155-9

Image Credits: AI Generated

DOI: 10.1038/s44160-026-01155-9

Keywords: nanographenes, helicenes, stereoselective synthesis, hexabenzocoronenes, graphene nanoribbons, chirality, chiroptical properties, circularly polarized luminescence, near-infrared emission, polycyclic aromatic hydrocarbons, optoelectronic materials, molecular synthesis